One of the most important characteristics of a laser diode is the beam divergence of the output beam in the far field. The smaller the far-field beam divergence, the higher the brightness of the output beam. Furthermore, in applications where the output beam is coupled into an optical fiber, smaller far-field beam divergence leads to higher coupling efficiency and improved alignment tolerance.
The beam divergence of the output beam in the far field is closely related to the beam width of the output beam in the near field. The larger the near-field beam width, the smaller the far-field beam divergence.
The optical power density of the output beam at an edge facet of the laser diode is, normally, limited to a peak optical power density, above which catastrophic optical mirror damage (COMD) occurs. Advantageously, a larger near-field beam width also allows a higher optical power for a given peak optical power density.
With reference to FIG. 1, a conventional high-power laser diode 100 has an asymmetric separate-confinement structure. The laser diode 100 comprises a first metal layer 110 and a second metal layer 111 of metallic material, for passing current to or from the laser diode 100, as well as a substrate 120 and a layer stack 130 of semiconductor material.
The layer stack 130 includes, in succession, a first cladding layer 140, for confining an optical mode, a first waveguide layer 150, for propagating the optical mode, an active layer 160, for generating the optical mode, a second waveguide layer 151, for propagating the optical mode, a second cladding layer 141, for confining the optical mode, and a contact layer 170, for facilitating ohmic contacting.
Typically, the substrate 120, the first cladding layer 140, and the first waveguide layer 150 are of n-type semiconductor material, the active layer 160 is of intrinsic semiconductor material, and the second waveguide layer 151, the second cladding layer 141, and the capping layer 170 are of p-type semiconductor material.
In FIG. 2, refractive index n and optical-mode intensity I are each plotted as a function of distance x from the second metal layer 111 along a vertical axis 190 through the laser diode 100 of FIG. 1. Within the layer stack 130, the active layer 160 and the contact layer 170 have respective refractive indices that are each greater than the respective refractive indices of the other layers in the layer stack 130. The first waveguide layer 150 and the second waveguide layer 151 have a substantially equal refractive index, and the first cladding layer 140 and the second cladding layer 141 have respective refractive indices that are each less than the refractive index of the first waveguide layer 150 and the second waveguide layer 151.
To reduce optical loss due to free-carrier absorption, the refractive index of the second cladding layer 141 is less than the refractive index of the first cladding layer 140. Furthermore, the second waveguide layer 151 has a thickness that is less than a thickness of the first waveguide layer 150. Such asymmetric refractive-index and thickness profiles skew the optical mode toward the portion of the layer stack 130 that is of n-type semiconductor material, which is desirable because free-hole absorption is more problematic than free-electron absorption.
The refractive-index profile of the layer stack 130 also ensures that the optical mode generated in the active layer 160 is largely confined by the first cladding layer 140 and the second cladding layer 141 to propagate mainly in the first waveguide layer 150, the second waveguide layer 151, and the active layer 160. The optical mode is emitted as an output beam from an edge facet of the laser diode 100 perpendicular to a horizontal plane of the active layer 160.
Typically, the vertical near-field beam width, that is, the near-field beam width in a direction perpendicular to the horizontal plane of the active layer 160, is much smaller than the horizontal near-field beam width, that is, the near-field beam width in a direction parallel to the horizontal plane of the active layer 160. Therefore, the vertical far-field beam divergence is much larger than the horizontal far-field beam divergence.
To decrease the vertical far-field beam divergence, a laser diode having a large vertical near-field beam width is desired. With reference to FIG. 3, one conventional approach to increasing the vertical near-field beam width of a laser diode 300 is to increase the thickness of a first waveguide layer 350 included in a layer stack 330 that is otherwise similar to that of the laser diode 100.
In FIG. 4, refractive index n and optical-mode intensity I are each plotted as a function of distance x from the second metal layer 111 along a vertical axis 390 through the laser diode 300 of FIG. 3. Comparison of FIG. 4 with FIG. 2 reveals that, in the laser diode 300, the vertical intensity profile of the optical mode is wider than that in the laser diode 100. Thus, the increased thickness of the first waveguide layer 350 leads to an increased vertical near-field beam width and a decreased vertical far-field beam divergence of the laser diode 300 relative to those of the laser diode 100. However, as a result, vertical overlap of the optical mode with the active layer 160 in the laser diode 300 is decreased relative to that in the laser diode 100, leading to a decreased vertical optical confinement factor of the laser diode 300 relative to that of the laser diode 100.
Another approach to increasing the vertical near-field beam width of a laser diode involves inserting a mode-attracting layer having a high refractive index into a layer stack similar to that of the laser diode 100. A mode-attracting layer may be inserted between a cladding layer and a waveguide layer, as disclosed in U.S. Pat. No. 6,961,358 to Erbert, et al., issued on Nov. 1, 2005; and in U.S. Pat. No. 6,522,677 to Petrescu-Prahova, et al., issued on Feb. 18, 2003; for example, which are incorporated herein by reference. Alternatively, a mode-attracting layer may be inserted between cladding layers, as disclosed in U.S. Pat. No. 6,987,788 to Kim, et al., issued on Jan. 17, 2006; and in U.S. Pat. No. 5,923,689 to Su, et al., issued on Jul. 13, 1999; for example, which are incorporated herein by reference.
Yet another approach to increasing the vertical near-field beam width of a laser diode involves inserting a mode-repelling layer having a low refractive index into a layer stack similar to that of the laser diode 100. A mode-repelling layer may be inserted between a cladding layer and a waveguide layer, as disclosed in U.S. Pat. No. 7,263,114 to Cho, issued on Aug. 28, 2007; and in U.S. Pat. No. 6,606,334 to Shigihara, et al., issued on Aug. 12, 2003; for example, which are incorporated herein by reference. Alternatively, a mode-repelling layer may be inserted between cladding layers, as disclosed in U.S. Pat. No. 5,289,484 to Hayakawa, issued on Feb. 22, 1994; and in U.S. Pat. No. 5,260,959 to Hayakawa, issued on Nov. 9, 1993; for example, which are incorporated herein by reference.
Yet another approach to increasing the vertical near-field beam width of a laser diode involves inserting both a mode-attracting layer having a high refractive index and a mode-repelling layer having a low refractive index into a layer stack similar to that of the laser diode 100. A mode-attracting layer and a mode-repelling layer may be inserted between a cladding layer and a waveguide layer having a graded refractive index, as disclosed in U.S. Pat. No. 7,251,381 to Buda, et al., issued on Jul. 31, 2007; in U.S. Pat. No. 6,993,053 to Buda, et al., issued on Jan. 31, 2006; and in U.S. Pat. No. 6,882,670 to Buda, et al., issued on Apr. 19, 2005; for example, which are incorporated herein by reference.
Although such prior-art laser diodes, generally, have a small vertical far-field beam divergence, many also have a small vertical optical confinement factor. An object of the present invention is to overcome the shortcomings of the prior art by providing a laser diode having both a small vertical far-field beam divergence and a large vertical optical confinement factor. In the laser diode of the present invention, a mode-splitting layer having a low refractive index is inserted between waveguide layers of a layer stack similar to that of the laser diode 100 to increase the vertical near-field beam width. The mode-splitting layer also produces a shoulder in an optical mode generated in an active layer of the layer stack, increasing vertical overlap of the optical mode with the active layer.